1. Field of the Invention
The present invention relates to a high efficiency solar cell that can be used as an energy source of an artificial satellite, etc. and, more particularly, a lattice matched solar cell using group III-V compound semiconductor, epitaxially grown on a germanium (Ge) substrate, and a method for manufacturing the same.
2. Description of the Related Art
Since the human beings are confronted by the serious energy problem, and in connection with the issue of global warming, the solar cell attracts a good deal of public attention, because the solar cell is supposed to be a clean energy source and to be environmental friendly, contrary to the fossil fuel, which emits carbon dioxide (CO.sub.2). However, it greatly depends upon its cost performance whether or not the solar cell can be put to practical and commercial use. That is, how to reduce the production cost is an extremely important factor in the solar cell.
The solar cell which employs the group III-V compound semiconductor such as gallium arsenide (GaAs) has a feature of the high efficiency, but contains the problem that cost of the group III-V compound semiconductor substrate is extremely high. For example, the GaAs substrate is a very expensive semiconductor substrate such that, although depending on the size and the grade, it exceeds hundred of dollars to a thousand dollar per wafer. For this reason, the spotlight of attention has been focused on the group III-V compound semiconductor solar cell in which the group III-V compound semiconductor is heteroepitaxially grown on the germanium (Ge) substrate, because Ge is relatively inexpensive and the heteroepitaxially grown structure has strong mechanical strength. Therefore, the GaAs/Ge heteroepitaxially solar cell has been known as the group III-V compound semiconductor solar cell epitaxially grown on the Ge substrate.
In addition, the tandem solar cell has also been known in which the indium gallium phosphide (InGaP) solar cell which is supposed to be lattice-matched with GaAs and has the wider band gap Eg than GaAs is further stacked on the GaAs/Ge heteroepitaxial solar cell. Each of the GaAs/Ge heteroepitaxial solar cell and the GaAs/Ge-GaAs/InGaP tandem solar cell is heteroepitaxially grown by the method such as the metalorganic chemical vapor deposition (MOCVD) method. In such heteroepitaxy, formation of the phase separation layer such as the antiphase domain due to the lattice mismatching between the surface lattice of the Ge substrate as the element semiconductor and the surface lattice consisting of gallium (Ga) and arsenic (As) atoms, etc. poses the problem. In order to avoid such formation of the phase separation layer, upon growing the GaAs layer on a surface of the Ge substrate, first the low temperature buffer layer having a small thickness of about 0.1 .mu.m is grown at the low temperature of less than 600.degree. C., and then a relatively thick (2 to 3 .mu.m) GaAs buffer layer is grown by increasing the substrate temperature up to the normal growth temperature (about 700.degree. C.). Then, after the low temperature buffer layer and the relatively thick GaAs buffer layer have been formed, a predetermined stacked structure needed to constitute the desired GaAs based solar cell or a stacked structure constituting the GaAs/InGaP tandem solar cell is grown.
In this case, it has been expected that the dislocation or the defect occurred on the Ge/GaAs heterojunction interface would be reduced by the above thin low temperature buffer layer and the relatively thick GaAs buffer layer. However, the good Ge/GaAs heteroepitaxy has not been obtained in practice. That is, it has been difficult to remove:
(i) the misfit dislocation caused due to the difference in the lattice constants between GaAs and Ge crystals, and
(ii) the dislocation caused due to the difference in thermal expansion coefficients between GaAs and Ge crystals in cooling process from the growth temperature.
In the prior art, the strained-layer superlattice, etc. are employed as the buffer layer and thus the slight effect of reducing the dislocation has been achieved, nevertheless they have not brought about the great improvement. It is guessed that the misfit dislocation in the above (i) is caused to relax the internal distortion, when the thickness of the GaAs layer, having the lattice mismatching rate of about 0.08% to the Ge substrate, exceeds about 0.3 to 0.5 .mu.m. Therefore, it is difficult for the GaAs solar cell layer, that needs its thickness of more than 2 to 3 .mu.m to absorb effectively the solar light, to prevent the generation of the misfit dislocation. In addition, since the dislocation of the grown layer caused in cooling process from the growth temperature to the room temperature, etc. stated in the above (ii) are due to difference between the thermal expansion coefficients (Ge: 5.5.times.10.sup.-6 K.sup.-1, GaAs: 6.0.times.10.sup.-6 K.sup.-1), it is difficult to prevent perfectly the generation of such dislocation, etc.